High purity salt aluminum flux

ABSTRACT

A salt flux composition comprising a high purity salt and additives is used in the recycle of scrap aluminum to increase the recovery of aluminum. The additives include an alkaline agent and a fluoride source.

FIELD OF THE INVENTION

[0001] This invention provides a composition and method of using thatcomposition in the recycle of aluminum, especially scrap aluminum suchas found in used beverage containers. More particularly, this inventionrelates to a high purity salt flux composition and additive compositionfor use with a high purity salt for making the high purity salt fluxcomposition, which salt flux composition can be used during theremelting of scrap aluminum. The method of the invention is directed tousing the high purity salt flux composition during the aluminum recoveryprocess to improve coalescence of the molten aluminum.

BACKGROUND OF THE INVENTION

[0002] Use of molten salt fluxes in the secondary aluminum industry isknown to improve direct recovery of aluminum in remelting processes.Aluminum and scrap aluminum, such as used beverage containers (UBCs),are treated using such processes. Remelting of the aluminum in a furnaceis carried out under cover of a layer of molten salt to preventoxidation of the aluminum in the furnace atmosphere and to promotecoalescence of the molten aluminum so as to maximize recovery ofaluminum. During processing, an oxide film tends to form on the surfaceof the molten aluminum droplets. The oxide film inhibits coalescence ofthe molten aluminum, causing smaller particles to be lost in the processthereby reducing the amount of aluminum recovered. The unrecoverablealuminum droplets having the oxide film are sometimes referred to asdross.

[0003] Use of a salt flux in the furnace helps to strip away and suspendthe oxide film so that coalescence of the droplets increases and drossformation decreases. The salt flux wets the oxide film and initiatesdisintegration of the film, stripping it from the surface of the moltenaluminum droplets. Fragments of the oxide film stripped from thealuminum remain suspended in the flux. The aluminum droplets, which havea density greater than the flux, then form a continuous molten padbeneath the flux layer. The flux also prevents further oxide formationby keeping the metal protected from the atmosphere of the furnace.

[0004] One type of salt flux is primarily composed of a mixture of highpurity sodium chloride and potassium chloride. The high purity saltsused in such processes are solution mined and purified by complex,highly developed methods.

[0005] An object of this invention is to provide for improved aluminumrecovery in a recycle process which includes scrap aluminum and saltflux.

[0006] Another object of the invention is to provide for improvedcoalescence of the molten aluminum in a process which includes meltingscrap aluminum with a high purity salt flux composition, especiallyaluminum from UBCs.

[0007] Yet another object of the invention is to use an additive with ahigh purity salt flux composition, which additive includes an alkalineagent and a fluoride source to increase the coalescence of the remeltedmolten aluminum, thereby improving recovery of the metal.

[0008] Further objects and advantages of the invention will be found byreference to the following specification.

SUMMARY OF THE INVENTION

[0009] The present invention is directed to a method and composition forincreasing aluminum recovery in the recycle of aluminum, and in animportant aspect, scrap aluminum such as aluminum from used beveragecontainers (UBCs). The salt flux composition of the invention protectsthe molten aluminum from oxidation, strips a protective oxide film fromthe molten aluminum so that molten aluminum droplets can coalescence andholds the oxide film in suspension so that the molten aluminum can berecovered.

[0010] Broadly, the salt flux composition of the invention compriseshigh purity NaCl and/or KCl, an alkaline agent and a fluoride source.The amounts of alkaline agent and fluoride source, along with the highpurity NaCl and/or KCl, are effective for improving coalescence andreducing aluminum loss in the recovery of aluminum from molten scrapaluminum, where the improvement is relative to a salt flux compositionwhich comprises a high purity salt without an alkaline agent and afluoride source.

[0011] In an important aspect, the invention includes a salt fluxcomposition which comprises high purity NaCl and/or KCl, an alkalineagent and a fluoride source, where the amounts of the alkaline agent andfluoride source, along with a high purity salt in the salt fluxcomposition, are effective for improving coalescence and reducingaluminum loss in the recovery of aluminum from molten scrap aluminum,especially UBCs which comprise specific alloys of aluminum. Such animprovement is relative to a process using like conditions and a highpurity salt flux composition consisting essentially of a high puritysalt without the alkaline agent and fluoride source. Generally, the saltflux composition comprises at least about 0.2 weight percent andpreferably from about 0.2 to about 3 weight percent alkaline agent andat least about 3 and preferably from about 3 to about 7 weight percentfluoride source, all based upon the weight of the salt flux composition.Generally, the salt flux composition comprises from about 90 to about96.8 weight percent NaCl and/or KCl in this aspect of the invention.

[0012] The high purity salt flux will have less than about 0.2 weightpercent sulfate. The composition and process of the invention also willat least maintain or improve aluminum coalescence as compared to aprocess using a salt flux consisting essentially of a high purity alkalimetal salt such as NaCl and/or KCl.

[0013] In another aspect, the invention is directed to an additivecomposition which consists essentially of an alkaline agent and fluoridesource for addition to high purity NaCl and/or KCl to provide a highpurity salt flux composition which improves aluminum coalescence in aprocess which involves mixing scrap aluminum metal, the additive andhigh purity salt to recover aluminum, as opposed to a process in whichaluminum is recovered from dross resulting from previous aluminumprocesses without the further addition of scrap aluminum.

[0014] In an important aspect, the alkaline agent is selected from thegroup consisting of Na₂CO₃, NaOH, KOH, K₂CO₃ and mixtures thereof andthe fluoride source is selected from the group consisting of MF, CaF₂,MAlF₄ M₃AlF₆ and mixtures thereof, where M is K or Na.

[0015] In another important aspect, the additive composition comprisesalkaline agent in an amount of from about 3 to about 50 weight percentand fluoride source in an amount of from about 50 to about 97 weightpercent, each based upon the weight of the additive composition. Inanother important aspect, the additive composition comprises betweenabout 4 to about 10 weight percent of the salt flux composition.

[0016] The salt flux composition of the invention generally is used inthe process of the invention at a level of at least about 1 weightpercent, based upon the weight of aluminum being processed.

DETAILED DESCRIPTION OF THE INVENTION Definitions

[0017] As used herein, “scrap aluminum” means aluminum stock left overfrom equipment or structural manufacture or used beverage cans.

[0018] As used herein, scrap aluminum from UBCs includes 3003 aluminumalloy, 3004 alloy and 5182 alloy.

[0019] As used herein, a “standard purity salt” means sodium chloridehaving at least about 0.3 weight percent sulfate or potassium chloridehaving at least about 0.02 weight percent sulfate, and a “high puritysalt” means sodium chloride having less than about 0.2 weight percentsulfate or potassium chloride having less than about 0.01 weight percentsulfate.

[0020] As used herein, “high purity salt flux composition” or “highpurity salt flux” means a flux composition which comprises a high puritysodium or potassium chloride or a mixture thereof. A high purity fluxcomposition has at least about the same or a lesser amount of sulfatethan which is in high purity NaCl, KCl or blends thereof, which is usedin the high purity flux composition.

[0021] As used herein, “additive composition” means a composition whichcomprises an alkaline agent and a fluoride source for use with an alkalimetal salt to provide a salt flux composition which may be used in therecovery of aluminum from scrap aluminum.

[0022] As used herein, “dross” means the formation of unrecoverablealuminum droplets having an oxide film covering the outer surface whichare entrapped within the salt flux layer in the furnace.

[0023] As used herein, “recovery yield” means the yield of recoveredaluminum metal from a recycling process where aluminum metal scrap ismixed with a high purity salt flux composition and where the yield ofrecovered aluminum is based upon the weight of the aluminum which is putinto the process as a starting material.

Salt Flux Composition

[0024] The invention provides a method and compositions for enhancedaluminum recovery when scrap aluminum is mixed with the high purity saltand additive and the blend is processed for recycle. The inventionprovides a high purity salt flux composition, which comprises highpurity NaCl and/or KCl, an alkaline agent and a fluoride source, to beused in a furnace during the recycle of aluminum, especially scrapaluminum such as found in used beverage containers (UBCs). The salt fluxcomposition of the invention is melted along with scrap aluminum, whichis not aluminum which is part of a dross blend, to provide a moltenmixture in the furnace, wherein the salt flux composition promotes thecoalescence of the molten aluminum droplets and prevents oxidation ofthe aluminum so as to increase the recovery yield of aluminum from therecycle process.

[0025] In an important aspect, the salt flux composition consistsessentially of high purity salt, alkaline agent and fluoride sourcewhich flux composition is mixed with scrap aluminum from UBCs comprisingalloys of aluminum, such as 3003, 3004 or 5182 aluminum alloys whichaluminum is not a part of any dross blend from previous aluminumprocessing. The UBCs also may contain up to about 2% magnesium.

[0026] In its broadest aspect, the salt flux composition of theinvention comprises high purity NaCl and/or KCl, an alkaline agent and afluoride source. The amounts of alkaline agent and fluoride source,along with the high purity NaCl and/or KCl, are effective for improvingcoalescence and reducing aluminum loss in the recovery of aluminum frommolten scrap aluminum, where the improvement is relative to a salt fluxcomposition which comprises a high purity salt without an alkaline agentand a fluoride source. Generally, in this aspect, the salt fluxcomposition comprises at least about 0.2 weight percent and preferablyfrom about 0.2 to about 3 weight percent alkaline agent and at leastabout 3 and preferably from about 3 to about 7 weight percent fluoridesource, all based upon the weight of the salt flux composition.Generally, the salt flux composition comprises from about 90 to about96.8 weight percent NaCl and/or KCl in this aspect of the invention.

[0027] When a mixture of high purity salts is used, the ratio of NaCl toKCl is from about 30:70 to 70:30. Preferably an essentially equimolarmixture of high purity NaCl and KCl is used in the high purity salt fluxto provide a lower melting temperature for the high purity salt fluxcomposition, as well as to lower the cost of the salt flux. Moreparticularly, it is desirable to provide a mixture of high purity saltshaving a composition at or near the eutectic point of the NaCl and KClblend so as to minimize melting temperature. It is possible, however, touse only NaCl or KCl with similar recovery results. Of course, thepresence of the alkaline agent and fluoride source in the salt fluxcomposition also will affect the melting temperature of the salt fluxcomposition. The melting point of the eutectic mixture of high puritysalts is about 750° C.

[0028] The additive composition of the invention is added to a highpurity salt to provide the salt flux composition of the invention. Theadditive composition comprises an alkaline agent and fluoride source inamounts effective for increasing the recovery yield of aluminum duringrecycle when the additive composition is added to a high purity alkalimetal salt. In an important aspect the additive consists essentially ofthe alkaline agent and fluoride source and the salt flux compositionconsists essentially of the high purity salt (NaCl and/or KCl), thealkaline agent and fluoride source. The additive composition comprisesat least about 4 weight percent of the high purity salt flux compositionand preferably between about 4 to about 10 weight percent.

[0029] The alkaline agent may be either high or low bulk density sodaash, K₂CO₃, NaOH or KOH. Preferably, the alkaline agent is soda ash.Preferably, the alkaline agent is added in an amount between about 0.2to about 3% based on the weight of the salt flux composition, althoughuse of an amount greater than about 2% provides only a limited increasein benefit.

[0030] While not intending to be bound by any theory, the alkaline agentincreases coalescence of the molten aluminum, even as compared tocertain salt fluxes comprising high purity salts. Another benefitassociated with the alkaline agent is that it promotes the formation ofthe non-reactive forms, CaO and MgO.

[0031] The fluoride source may be KF, NaF, CaF₂, Na₃AlF₆ (cryolite),K₃AlF₆, NaAlF₄ (SATF), KAlF₄ or mixtures thereof. Preferably, it iscryolite or SATF. While not intending to be bound by any theory, afluoride source in the additive composition improves coalescence of themolten aluminum by increasing the dissolution of the oxide film on themolten aluminum droplets. The fluoride source is in an amount effectivefor improving coalescence of the molten aluminum. Preferably, thefluoride source is present in an amount between about 3% to about 7%based on the weight of the salt flux composition. Addition of a fluoridesource and/or the alkaline agent to the high purity salt flux also mayreduce the melting temperature of the high purity flux composition.

Method of the Invention

[0032] The method of the invention is directed to enhanced recovery ofaluminum in a remelting process. If UBCs are to be recycled,pre-processing includes the mechanical shredding of the UBCs into stripsof about 12 inches long and about ½to about 1 inch wide. If other formsof aluminum are to be processed according to the method of theinvention, they should be pre-processed, if necessary, to providesimilarly sized particles. Preferably, the aluminum also is delacquered,if necessary. Any method known to one skilled in the art can be used toshred and to delacquer the aluminum in preparation for the remeltingprocess.

[0033] Preferably, the components of the salt flux composition of theinvention or a high purity salt and the additive are combined to form adry mixture prior to being charged to a furnace. The salt fluxcomposition then is charged to the furnace, such as a vertical muffle orrotary furnace or other suitable, commercially available furnace, formelting either prior to or concurrently with addition of the aluminum.The furnace should have as its melting zone a container that isrelatively inert to the molten salt flux so that impurities are notintroduced into the flux composition from the container. The temperatureof the furnace is held between about 750° C. and about 800° C. The saltflux composition is melted at from about 740° C. to about 750° C. andmay be held in the molten state for about 300 minutes. Preferably, areducing atmosphere is maintained in the furnace to reduce oxidation ofaluminum to aluminum oxide.

[0034] The shredded scrap metal then is added to the molten salt fluxcomposition in the furnace as a batch process. The purified aluminumwhich has coalesced beneath the salt flux layer is decanted from thefurnace after approximately 30 minutes. Generally, the recovery yield ofaluminum decreases the longer the aluminum remains in the furnace. Theprocess also may be modified for continuous processing of the scrapaluminum.

[0035] The amount of flux composition used in the furnace is at leastabout 1 weight percent and, in an important aspect, is from about 1% toabout 50%, based upon the weight of aluminum. Preferably, the amount offlux used in the process is from about 2% to about 5% based upon theweight of the aluminum. With each batch of aluminum processed, fragmentsof oxide film, particles of aluminum coated with the oxide film andother impurities become entrapped in the flux composition layer, causingit to become more cloudy and viscous. The salt flux composition may bere-used in the furnace until the flux composition becomes too viscous,which makes it difficult to remove purified aluminum. Generally, thesalt flux composition may be re-used approximately 6 times. After theinitial charge of salt flux composition, an amount of flux compositionis added to the furnace along with each batch of aluminum for eachre-use of the flux composition. Approximately 5% to 15% of the totalweight of flux composition initially charged to the furnace is addedwith each subsequent batch of aluminum.

[0036] The following example illustrates a method for carrying out theinvention and should be understood to be illustrative of, but notlimiting upon, the scope of the invention which is defined in theappended claims.

EXAMPLE 1

[0037] Various flux compositions, as described below in Table 1, weretested in duplicate in this experiment. Compositions 1 and 2 wereincluded in the experiment as control compositions. Compositions 3-6relate to the invention. In all compositions, the fluoride source wasSATF, the carbon source was coal and the alkaline agent was soda ash.SATF was added to the salt flux composition in an amount of about 5weight percent, based on the weight of the salt flux composition. Sodaash, when used, was added to the salt flux composition in an amount ofabout 1 weight percent, based on the weight of the salt fluxcomposition. TABLE 1 Composition Components 1 High Purity Salt +Fluoride Source 2 High Purity Salt + Fluoride Source 3 High PuritySalt + Fluoride Source + Carbon Source + Alkaline Agent 4 High PuritySalt + Fluoride Source + Carbon Source + Alkaline Agent 5 High PuritySalt + Fluoride Source + Alkaline Salt 6 High Purity Salt + FluorideSource + Alkaline Agent

[0038] Forty-five grams of flux composition was heated in a crucible andallowed to melt for 20 minutes. After 20 minutes, 2.25 grams ofdelacquered, shredded aluminum beverage cans was added to the moltenflux. The flux with aluminum was maintained in a molten state at 800° C.for one hour without agitation. The crucible was inclined at a 20° slopefrom the horizontal to bring the molten aluminum drops into contact withone another. The crucible then was cooled to room temperature. Watersoluble material was removed by dissolving in water. The remaining waterinsoluble materials, including aluminum, were dried. The aluminumparticles were removed.

[0039] A visual inspection of the recovered aluminum revealed thatcoalescence is improved with a salt flux composition comprising highpurity salt, an alkaline agent and fluoride source as compared to a highpurity salt flux composition without the additive. The use of a carbonsource in the salt flux composition provides no visible improvement tocoalescence when a high purity salt flux composition with alkaline agentand fluoride source is used.

What is claimed is:
 1. A salt flux composition effective for use in therecovery of scrap aluminum comprising: a high purity alkali metal saltselected from the group consisting of NaCl, KCl and mixtures thereof; analkaline agent; and a fluoride source, the alkaline agent and fluoridesource each in amounts effective for improving the recovery of aluminumin a process which includes the recovery of aluminum from remeltedmolten scrap aluminum as compared to a process which utilizes a saltflux composition consisting essentially of a high purity alkali metalsalt without the alkaline agent and fluoride source.
 2. A salt fluxcomposition as recited in claim 1 wherein the salt flux compositioncomprises: from about 0.2 to about 3 weight percent alkaline agent,based upon the weight of the salt flux composition, and from about 3 toabout 7 weight percent fluoride source, based upon the weight of thesalt flux composition.
 3. A salt flux composition as recited in claims 1or 2 wherein the salt flux composition comprises at least about 90weight percent alkali metal salt.
 4. An aluminum salt flux compositionas recited in claim 1 wherein the alkaline agent and fluoride source arepresent in amounts effective for improving the recovery of aluminum whenthe salt flux composition is mixed with molten aluminum and the alkalimetal salt is present at a level of at least about 1 weight percentbased upon the weight of the molten aluminum.
 5. A salt flux compositionas recited in claims 1 or 2 wherein the fluoride source is selected fromthe group consisting of MF, CaF₂, MAlF₄, M₃AlF₆, and mixtures thereofwhere M is sodium or potassium.
 6. A salt flux composition as recited inclaim 4 wherein the fluoride source is selected from the groupconsisting of MF, CaF₂, MAlF₄, M₃AlF₆ and mixtures thereof where M issodium or potassium.
 7. A salt flux composition as recited in claims 1or 2 wherein the alkaline agent is selected from the group consisting ofNa₂CO₃, NaOH, KOH, K₂CO₃ and mixtures thereof.
 8. A salt fluxcomposition as recited in claim 6 wherein the alkaline agent is selectedfrom the group consisting of Na₂CO₃, NaOH, KOH, K₂CO₃ and mixturesthereof.
 9. An additive composition for a salt flux, the additivecomposition consisting essentially of: an alkaline agent, and a fluoridesource, the alkaline agent and fluoride source each in amounts effectivefor improving the recovery of aluminum in a process which includes therecovery of aluminum from remelted molten aluminum when the additive ismixed with the molten aluminum and an alkali metal salt selected fromthe group consisting of NaCl, KCl and mixtures thereof, as compared to aprocess which utilizes a salt flux composition consisting essentially ofa high purity alkali metal salt wherein the alkali metal salt isselected from the group consisting of NaCl, KCl and mixtures thereof.10. An additive composition as recited in claim 9 wherein the alkalineagent is present in an amount of from about 3 to about 50 weight percentbased upon the weight of the additive composition and the fluoridesource is present in an amount of from about 50 to about 97 weightpercent, based upon the weight of the additive composition.
 11. Anadditive composition as recited in claims 9 or 10 wherein the fluoridesource is selected from the group consisting of MF, CaF₂, MAlF₄, M₃AlF₆and mixtures thereof where M is sodium or potassium.
 12. An additivecomposition as recited in claim 11 wherein the alkaline agent isselected from the group consisting of Na₂CO₃, NaOH, KOH, K₂CO₃ andmixtures thereof.
 13. A process for the recovery of aluminum from scrapaluminum to increase the coalescence of molten aluminum during theprocess, the process comprising: mixing the scrap aluminum in a solid ormolten form and a salt flux composition in a solid or molten form toprovide a molten mixture, the salt flux composition comprising: a highpurity alkali metal salt selected from the group consisting of NaCl, KCland mixtures thereof; an alkaline agent; and a fluoride source, thealkaline agent and fluoride source each in amounts effective forimproving the recovery of aluminum in the process as compared to aprocess which utilizes a salt flux composition consisting essentially ofa high purity alkali metal salt without the alkaline agent and fluoridesource.
 14. A process as recited in claim 13 wherein the fluoride sourceis selected from the group consisting of MF, CaF₂, MAlF₄, M₃AlF₆ andmixtures thereof where M is sodium or potassium.
 15. A process asrecited in claims 13 or 14 wherein the alkaline agent is selected fromthe group consisting of Na₂CO₃, NaOH, KOH, K₂CO₃ and mixtures thereof.16. A process as recited in claim 15 wherein the salt flux compositioncomprises: from about 0.2 to about 3 weight percent alkaline agent,based upon the weight of the salt flux composition, and from about 3 toabout 7 weight percent fluoride source, based upon the weight of thesalt flux composition.
 17. A process as recited in claim 16 wherein thealkaline agent and fluoride source are present in amounts effective forimproving the recovery of aluminum when the salt flux composition ismixed with molten aluminum and the alkali metal salt is present at alevel of at least about 1 weight percent based upon the weight of themolten aluminum.
 18. A salt flux composition effective for use in therecovery of scrap aluminum consisting essentially of: at least about 90weight percent of an alkali metal salt selected from the groupconsisting of NaCl, KCl and mixtures thereof; at least about 0.2 weightpercent alkaline agent, based upon the weight of the salt fluxcomposition, the alkaline agent being selected from the group consistingof Na₂CO₃, NaOH, KOH, K₂CO₃ and mixtures thereof; and at least about 3weight percent fluoride source, based upon the weight of the salt fluxcomposition, the fluoride source being selected from the groupconsisting of MF, CaF₂, MAlF₄, M₃AlF₆ and mixtures thereof where M issodium or potassium.
 19. A salt flux composition as recited in claim 18wherein the salt flux composition comprises: from about 0.2 to about 3weight percent alkaline agent, based upon the weight of the salt fluxcomposition, and from about 3 to about 7 weight percent fluoride source,based upon the weight of the salt flux composition.
 20. A salt fluxcomposition as recited in claim 18 wherein the alkaline agent andfluoride source are present in amounts effective for improving therecovery of aluminum when the salt flux composition is mixed with moltenaluminum and the alkali metal salt is present at a level of at leastabout 1 weight percent based upon the weight of the molten aluminum. 21.A process for the recovery of aluminum from scrap aluminum and forincreasing the coalescence of aluminum during the process, the processcomprising: mixing the scrap aluminum in a solid or molten form and asalt flux composition in a solid or molten form to provide a moltenmixture, the salt flux composition consisting essentially of: at leastabout 90 weight percent of an alkali metal salt selected from the groupconsisting of NaCl, KCl and mixtures thereof; at least about 0.2 weightpercent alkaline agent, based upon the weight of the salt fluxcomposition, the alkaline agent being selected from the group consistingof Na₂CO₃, NaOH, KOH, K₂CO₃ and mixtures thereof; and at least about 3weight percent fluoride source, based upon the weight of the salt fluxcomposition, the fluoride source being selected from the groupconsisting of MF, CaF₂, MAlF₄, M₃AlF₆ and mixtures thereof where M issodium or potassium.
 22. A process as recited in claim 21 wherein thesalt flux composition consists essentially of: from about 90 to about96.8 weight percent alkali metal salt, based upon the weight of the saltflux composition, from about 0.2 to about 3 weight percent alkalineagent, based upon the weight of the salt flux composition, and fromabout 3 to about 7 weight percent fluoride source, based upon the weightof the salt flux composition.